[llvm] [AArch64] Make IFUNC opt-in rather than opt-out. (PR #171648)

[Daniil Kovalev via llvm-commits]

https://github.com/kovdan01 approved this pull request.

LGTM, thanks! This resolves the following linker errors which started to appear after #133533 when building libcxx with signed GOT enabled:

```
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__113basic_istreamIcNS_11char_traitsIcEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__113basic_ostreamIcNS_11char_traitsIcEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__114basic_iostreamIcNS_11char_traitsIcEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__113basic_istreamIwNS_11char_traitsIwEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__113basic_ostreamIwNS_11char_traitsIwEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__114basic_ifstreamIcNS_11char_traitsIcEEEE' requested, but only one type of GOT entry per symbol is supported
ld.lld: error: both AUTH and non-AUTH GOT entries for '_ZTVNSt3__114basic_ofstreamIcNS_11char_traitsIcEEEE' requested, but only one type of GOT entry per symbol is supported
```

@pcc I would appreciate if you could take a look at this as soon as you can since your changes from #133533 unfortunately break pauthtest toolchain build with signed GOT enabled, and this PR resolves the issue.

https://github.com/llvm/llvm-project/pull/171648